Native lignin is the second most abundant natural polymer on Earth. It is an irregular heterogeneous polymer. It is widely believed that lignin structure is tridimensional; however, concrete evidence is lacking, which causes some scientists to question whether the structure truly is tridimensional (Ralph et al. (2004)). Lignin is optically inactive. The repeatable (monomeric) unit in lignin is the phenylpropane unit (or the so-called C9-unit) of the p-hydroxyphenyl (H), guaiacyl (G) and syringyl (S) types (FIG. 1). Coniferous lignins are predominantly of the G-type. Hardwood lignins contain both G-units and S-units. The H-unit content in woody lignin is usually low; however, the H-unit content can significantly contribute to the structure of non-woody lignins (for instance, lignins derived from annual fibers). In addition, annual fiber lignins contain significant amounts of cinnamic and ferrulic acid derivatives attached to the lignin predominantly by ester linkages with the gamma hydroxyl of the C9-units (Ralph et al. (2004), Adler (1977), and Sakakibara (1991)).
Lignin C9-units contain different functional groups. The most common functional groups are aromatic methoxyl and phenolic hydroxyl, primary and secondary aliphatic hydroxyls, small amounts of carbonyl groups (of the aldehyde and ketone types) and carboxyl groups. The monomeric C9 lignin units are linked together to form the polymeric structure of lignin via C—O—C and C—C linkages. The most abundant lignin inter-unit linkage is the β-O-4 type of linkage (see structures 1-4 and 7 of FIG. 1). They constitute about 50% of the inter-unit linkages in lignin (about 45% in softwoods, and up to 60-65% in hardwoods). Other common lignin inter-unit linkages are the resinol (β-β) (structure 6), phenylcoumaran (β-5) (structure 5), 5-5 (structure 12) and 4-O-5 (structure 11) moieties. Their number varies in different lignins, but typically does not exceed 10% of the total lignin moieties. The number of other lignin moieties is usually below 5%. (Adler (1977), Sakakibara (1991), Balakshin et al. (2008))
The degree of lignin condensation (“DC”) is an important lignin characteristic, as it is often negatively correlated with lignin reactivity. The definition of condensed lignin moieties is not always clear. Most commonly, condensed lignin structures are lignin moieties linked to other lignin units via the 2, 5 or 6 positions of the aromatic ring (in H-units also via the C-3 position). The most common condensed structures are 5-5′, β-5 and 4-O-5′ structures. Since the C-5 position of the syringyl aromatic ring is occupied by a methoxyl group, and therefore it cannot be involved in condensation, hardwood lignins are typically less condensed than softwood lignins.
According to the current understanding in the field, typically most lignin in softwood and softwood pulps is linked (i.e., chemically attached) to polysaccharides, mainly hemicelluloses (Lawoko et al. (2005)). The main types of lignin-carbohydrate (“LC”) linkages in wood are phenyl glycoside bonds (structure A), esters (structure B) and benzyl ethers (structure C) (see FIG. 1) (Koshijima et al. (2003), Helm (2000), and Balakshin et al. (2007)). The occurrence of stable LC bonds is one of the main reasons preventing selective separation of the wood components in biorefining processes.
Technical lignins are obtained as a result of lignocellulosic biomass processing. Technical lignins differ dramatically from lignin in its native, natural form found in nature (so-called “native lignin”) as a result of the combination of multiple reactions that take place during biomass processing. These reactions can include catalyzed biomass hydrolysis, condensation of extracted lignin fragments, elimination of native lignin functional groups, formation of new functional groups, and others. Technical lignins are appreciably more heterogeneous (in terms of chemical structure and molecular mass) than the native lignins. Technical lignins have a large variety of structural moieties typically present in rather small amounts (Balakshin et al. (2003), and Liitia et al. (2003)).
In terms of the chemical structure, native lignins undergo significant degradation and/or modification during biomass processing. Lignin degradation occurs predominantly via cleavage of β-O-4 linkages (although the mechanisms typically are different for different processes) that results in an increase in the amount of phenolic hydroxyls and a decrease in the molecular mass of lignin. Lignin degradation also leads to a decrease in the amount of aliphatic hydroxyls, oxygenated aliphatic moieties and the formation of carboxyl groups and saturated aliphatic structures. In contrast to lignin degradation, some reverse reactions, such as lignin re-polymerization and/or re-condensation, typically take place to certain extents. These reverse reactions typically result in an increase in the molecular mass of lignin, and a decrease in its reactivity. These changes are common for most of the technical lignins, although the degree of transformation can vary significantly depending on the process conditions (temperature, time, pH, and others), feedstock origin, and feedstock identity.
Each process typically provides the lignin with some specific chemical characteristics. First, the reaction mechanism can be quite different in acidic and base media. The cleavage of β-O-4 linkages under alkaline conditions occurs via a quinone-methide intermediate and results in formation of coniferyl alcohol type moieties as a primary reaction product (FIG. 2). They are not accumulated in the lignin however, but undergo further secondary re-arrangement reactions forming various (aryl-) aliphatic acids. β-5 and β-1 type of linkages in native lignin typically cannot be cleaved during the process but are transformed into stilbene type structures (structure 30, FIG. 1). The latter are stable and are accumulated in alkaline lignins. In addition, significant amount of vinyl ether structures (structure 29, FIGS. 1 and 2) are formed during soda pulping and accumulated in the lignin, which is in contrast to kraft lignin. Another structural difference between soda and kraft lignins, as a result of difference in the reaction mechanism, is the presence of aryl-glycerol type structures (structure 20, FIG. 1) in soda lignins. On the other hand, lignin undergoes a demethylation reaction resulting in formation of o-quinone structures during kraft pulping (but not during soda pulping). In addition, kraft lignins typically contain a few percent of organically bonded sulfur, likely in the form of thiol compounds (Balakshin et al. (2003), Gellerstedt (1996), Marton (1971), and Geirer (1980)). Kraft and soda lignins have a significantly higher degree of condensation than the corresponding native lignins. However, this typically is the result of accumulation of condensed moieties of the original native lignin, rather than the extensive condensation reactions during pulping (Balakshin et al. (2003)). Kraft and soda lignins typically contain a few percent of carbohydrate and ash impurities. The amounts of these contaminants are also dependent on the feedstock origin, and the amount of contaminants is typically significantly higher in the annual fiber lignins than in woody lignins.
There is a large variety of lignins that may originate from potential acid-based biorefinery processes (Glasser et al. (1983)). The acid-base processes may be performed by the addition of mineral or organic acids (anywhere from catalytic amounts, up to the use of organic acids as the reaction media) or without acid addition (e.g., autohydrolysis), in which organic acids are generated due to the cleavage of acetyl groups contained in lignocellulosic biomass. Autohydrolysis may also occur due to the formation of acidic reaction products. Technical lignins derived from potential biorefinery processes are much less investigated than kraft lignins.
The main pathway of lignin degradation under acidic condition is the acidic hydrolysis of β-O-4 linkages (FIG. 3). The major product of this reaction is the so-called Hibbert's ketones (Wallis (1971)). Accumulation of these moieties in lignin results in relatively high content of carbonyl groups and the corresponding saturated aliphatic structures, as compared to alkaline lignins (Berlin et al. (2006)). Although degradation of lignin under acidic condition typically occurs via vinyl ether intermediates, these vinyl ether intermediates typically do not accumulate in the lignin, because vinyl ether structures typically are very unstable in acidic media. Significant amounts of olephinic moieties were observed in lignin obtained under acidic conditions, but their nature is different from the olephinic structures of kraft and soda lignins. Their exact structure is still not well understood (Berlin et al. (2006)).
Moreover, lignin condensations taking place under acidic conditions are typically more significant than those taking place in the alkaline process. Acidic lignin condensation occurs predominantly via the 2,6 position of the aromatic ring, in contrast to alkaline condensation which occurs predominantly at the C-5 position of the aromatic ring (Glasser et al. (1983)). The degree of lignin condensation typically is dependent on the acidity of the reaction media (pH and solvent used) and the process severity (temperature, time, pressure). As an extreme example, the most modified technical lignin known is the industrial acid hydrolysis lignin produced in Russia obtained at 170-190° C. for 2-3 h with 1% H2SO4. This modified technical lignin is highly insoluble in polar organic solvents and NaOH solution due to strong condensation/polymerization reactions during the process. It also has a relatively high content of phenolic hydroxyl groups and olephinic structures. In addition, the modified technical lignin contains 10-30% residual carbohydrates and up to 20% lipophilic extractives (Chudakov (1983)). In contrast, a significant fraction (70-90%) of acid hydrolysis lignin obtained at very high reaction temperature (greater than 2200° C.) but short reaction time (less than 1 min.) was soluble in a solution of 50% dioxane in 1N NaOH. The carbohydrate content in these soluble lignins was significantly lower at 2-4% (Glasser et al. (1983), and Lora et al. (2002)). Steam explosion lignin is also quite degraded in terms of cleavage of β-O-4 linkages, but typically much less condensed than acid hydrolysis lignins (Glasser et al. (1983), Robert et al. (1988), and Li et al. (2009)). In addition to structural variations in lignins obtained by different processes (“between-process” variations), there are also typically some structural differences between lignins obtained within the same type of process lignin (“within-process” variations). For example, one of the more important factors in within-process variations is the feedstock origin. This directly typically affects structural characteristics, such as ratios between the S, G, and H units, as well as the degree of condensation. It has been shown that various hardwood lignins can degrade differently during kraft pulping, which typically results in different hydroxyl and carboxyl group content, and different β-O-4, β-β, and β-5 linkages (Capanema et al. (2005)).
Lignins having unique structures also have unique properties, and these structurally unique lignins have uses in many different fields and applications, including, for example, adhesives and plastics. Thus, there remains a need in the art for high purity lignins and lignin compositions having unique structures and properties. The application is directed to these, and other important needs.